The following steps are performed in the control method: determining a mode of operation from amongst a permanent mode and a transient mode, as a function of a set of variables comprising said estimated values; correcting the value of the speed of rotation of the outlet shaft in such a manner that: if the mode has been determined as being the permanent mode, then the moving average (L') of the gear ratio (L) over a period (T) of a plurality of unit time intervals lies between a first threshold value (S.sub.1) that is negative and a second threshold value (S.sub.2) that is positive; and if the mode has been determined as being the transient mode, then said moving average (L') of the gear ratio (L) lies outside the range of values defined by the first and second threshold value (S.sub.1, S.sub.2).